Micrototal analysis systems of the prior art have typically required integrated sensors to be positioned in the fluidic circuit for the control and analysis of cells. Most of those prior art sensors use optical detection in the form of fluorescence spectroscopy, surface plasmon resonance, surface enhanced raman scattering, radiological detection or impedance measurements. Sensors based on impedance measurements can, in at least some applications, be advantageous because they are relatively simple and can be miniaturized for easy integration into a catheter, implantable device, etc. However, impedance measurements of biomaterials traditionally have very broad spectral responses that cannot be used for distinguishing different biospecies.
When electrolytic fluids flow in a microchannel under laminar flow conditions, a parabolic velocity profile exists such that ions in the middle of the microchannel travel faster than ions near the walls of the microchannel. Thus ions become redistributed in an electric double layer (EDL) within the microchannel. Delivery of ac voltage across the microchannel channel electrodes causes the ions to move back and forth across the electrodes. Electrokinetic effects develop as a result of the ionic redistribution and such electrokinetic effects contribute to changes in electric admittance. Thus the flow of fluid is very sensitive to the admittance across microelectrodes and in the microchannel. Thus, measuring the increase in electrical admittance can precisely account for the flow rate of the electrolytic fluid.
The electrical admittance of a liquid and particles suspended in a liquid increases when they are passed across channel electrodes. This increase of admittance shows spectral behavior with the stimulating electrical signal. The flow induced spectral response shows a characteristic signature for different cells or particles. Interaction between the electrical and viscous stress on the fluid is the main cause that gives rise to spectroscopic behavior. A critical frequency is defined as:
      f    c    =      σ          2      ⁢      πɛ      where σ and ε are the electrical properties of conductivity and permittivity of the buffer solution, respectively. For f<fc, the electrical field functions in a resistive manner, while the double layer functions in a capacitive manner.
Thus, there exists a potential for development of devices and methods which utilize electrical admittance as the basis for measuring fluid flowrates and/or for distinguishing biospecies (e.g., cell characterization, cell sorting, distinguishing different biochemicals, etc.).